For different diseases and injuries of a hip joint, so-called total endoprostheses are known which consist of two parts. One part is a shell, consisting of a plastic material which is generally compatible with tissue, wherein the shell is cemented in the hip pan after previous cutting. The other part is a prosthesis portion, being longitudinal and relatively large, and consisting generally of a suitable steel alloy. At the end of the prosthesis portion there is arranged a sphere which substitutes for the femur head of the injured bone. The sphere passes over a transition part and over a flange into a curved shaft, which is inserted in the bone, after previous separation of the top of the femur neck, and is anchored in the bone with bone cement.
With these known total endoprostheses, positive results have been achieved, in comparison with the previously known possibilities for treatment of such fractures and diseases, like arthroses and the like. However, the complication rate is unsatisfactory due to the relatively radical operation of the body which is to be treated. During such an operation, a considerable amount of bone has to be removed which, under circumstances such as wear phenomena of the hip joint, is not injured but is unusable for the planned bearing function. This results in the use of other treating methods in the case of wear phenomena such as arthroses and the like.
Therefore, these radical operations are avoided, if they are not, because of certain fractures, absolutely necessary. Especially in the case of wear phenomena of joints, attempts have been made to substitute for the sliding areas which are especially important for the function of the respective joint and are, simultaneously, subject to great loads, without removing its supporting substructure. This has been accomplished, for example, by cementing a shell-shaped pan of suitable plastic material or ceramic in a hip joint pan similar to the previously described total endoprosthesis. Furthermore, the counter-sliding surface for the artificial hip joint pan has also been substituted for by a shell, consisting of metal, namely, of a suitable steel. This surface is shaped on its outer side as a spherical segment, whose vertical height is generally somewhat smaller than its transverse dimension, i.e., smaller than the radius of curvature of the outer side of the segment.
To attach such a metal shell on the femur head, the end face of the femur head has to be adapted to the curve of the metal shell. Moreover, the femur head has to be treated in extension of its end face, i.e., on its jackets, in order to adapt the femur head to the corresponding dimensions of such a shell prosthesis.
It turns out now that these necessary treatments of a bone which is to be provided with a shell prosthesis, are relatively time-consuming and, therefore, extend the operation time.
Furthermore, there is the particular disadvantage that there cannot be produced precise fitting areas when cutting the sphere-shaped and the cylindrical end sections of the respective bone portions, because the freely guided instrumentation is not able to fulfill this requirement. Moreover, during the treatment of the bone, the instrumentation often removes tissue material at a point where, due to the existing strength, there is an especially small treatment resistance. At such points, however, there is no need to remove any or, at least, not so much tissue material. Such a free treatment of a bone leads, however, not only to imprecise fitting areas but, there is also basically no possibility to arrange the implant physiologically in axis alignment, which is particularly desirable especially in highly stressed joints.
Furthermore, free treatment of a bone is also of disadvantage, when several operation steps are performed one after another because, during a subsequent treatment step, possibly satisfactory intermediate results can be diminished with regard to other fitting areas or the like.